Plasma lipoproteins

Lipoproteins (or lipoprotids) are macromolecules deriving from the combination of a protein with lipids of various kinds: cholesterol, cholesterol esters, phospholipids and triglycerides. The fatty substances, insoluble in a watery environment, can in fact be transported into the blood stream only if they are linked to specific lipoproteins. The following table shows their main features:

Lipoproteins *	chylomicrons	VLDL	LDL	HDL
Density (g / ml)	0.93	0.95-1.006	1019-1063	1063-1210
Diameter Å *	800-5000	300-800	216	74-100
Composition: protein% lipid%	<2 98	8 ninety two	22 78	50 50
Lipid major	Triglycerides	Triglycerides	Cholesterol	Cholesterol
Main function	Transport of exogenous triglycerides (taken with food)	Transport of endogenous triglycerides (synthesized by the body)	Transport of cholesterol to peripheral tissues	Cholesterol transport from peripheral liver tissues
Origin	Intestine	Liver	VLDL metabolism	Intestine Liver

In addition to these, which are the best known, there is a fifth type of plasma lipoprotein, which however does not reach significant concentrations due to its rapid turnover. These are IDLs, or intermediate-density lipoproteins, which, produced due to the degradation of chylomicrons and VLDL (therefore contain almost equal triglycerides and cholesterol), are also known as "remnants", ie "remaining" from degradation of other lipoproteins.

Lipoprotein functions

The best known lipoproteins are those of the plasma, deputed to the transport of lipids from the intestine to the liver and from the liver to the various tissues.

KILOMYCHRONES: lipoproteins consisting of a lipid heart (formed by triglycerides, phospholipids, cholesterol and fat-soluble vitamins), surrounded by protein molecules. This sort of mantle, thanks to the water solubility conferred by the proteins, increases the solubility degree of the chylomicrone in the aqueous medium. These macromolecules are produced in enterocytes (intestinal cells), from which they escape to enter the lymphatic circulation and, subsequently, into the bloodstream. After distributing their lipid content to the various tissues, the chylomicrons are conveyed to the liver, which then destroys their protein shell and recycles the lipid residues contained within them.

VLDL (very low density lipoproteins) are synthesized by hepatocytes. They transport triglycerides from the liver (where they were synthesized, for example, from glucose) to other tissues (especially the adipose and muscular tissues).

LDL (low density lipoprotein) derives from VLDL, due to the progressive depletion of their triglyceride content. They are loaded with cholesterol that transport and distribute to peripheral tissues.

HDL (high density lipoprotein) is secreted into the blood from the liver and intestines. They are therefore deputed to the transport of cholesterol from the peripheral tissues to the liver (they carry out the so-called reverse cholesterol transport).